The structure of Al2O3 implanted with iron at 77 K

144

Nuclear

THE STRUCTURE C.J. McHARGUE, Oak Ridge Nutional

A. PEREZ

OF AI,O, IMPLANTED P.S. SKLAD,

Lahoraro~,

Instruments

and Methods

in Physics

Research

846 (1990) 1444148 North-Holland

WITH IRON AT 77 K *

J.C. McCALLUM

and C.W. WHITE

Oak Rtdge. Til;. USA 37831-6118

and G. MAREST

C’nrwrsit~ Chude

Bernard Lyon I, 69622 V~lleurhanne Cede.u. France

Implantation of iron in the fluence range of 1x10” to 1 x10” ions/cm’ (160 keV) produces an amorphous surface layer in Al zO, if the substrate temperature is approximately 77 K during implantation. Conversion electron Mossbauer spectroscopy (CEMS) measurements on the implanted “Fe show a distribution of the iron among the charge states of Fe’+, Fe”, and the unusual Fe4’ state. The Miissbauer parameters indicate that there are two Fe” and two Fe4+ states present in amounts that vary with fluence. No Fe’+ was detected. The fine structure of the electron energy loss spectra indicates that the short-range order in amorphous Fe-implanted Al>O, differs from that in amorphous Al?O, created by stoichiometric (2 Al + 3 0) implantation.

1. Introduction The defect structure of ion implanted insulators is complex both due to the damage-producing processes and due to constraint imposed on the residual damage state by the chemical bonding forces. The defects that survive annihilation during the postcascade cooldown may rearrange to form clusters or complexes with either the injected ions or the host atoms. In general, the crystal structures require that local electrical charge neutrality be maintained, giving rise to charged point defects or impurity-point defect complexes. The Mossbauer effect provides a unique probe for investigating the structural aspects of solids on the atomic scale. Measurement of the hyperfine interactions provide information on symmetry, ordering. and chemical bonding in the immediate environment of the probing nucleus. Conversion electron Mossbatter spectroscopy (CEMS) is particularly suited for probing the near-surface region to a depth typical of the range of implanted ions [l]. The CEMS technique has been used to determine the charge states of iron implanted into MgO [2], LiF [3]. TiO, [4]. or AI,O, at room temperature [5]. In these studies. the host material remained crystalline during implantation. This paper reports a study of amorphous AlzO, produced by implantation of iron at 77 K and compares

* Research sponsored in part by the Division of Materials Sciences. US Department of Energy, under contract DEAC05-840R21400 with the Martm Marietta Energy Systems. Inc. 0168-583X/90/$03.50

(North-Holland)

?J Elsevier Science Publishers

B.V.

the results to a similar study in which the implantation temperature was about 300 K and in which the Al,O, remained crystalline [5].

2. Experimental

conditions

High-purity AI,O, single crystals having (0001) normal to the surface were given an optical grade polish and then annealed 120 h at 1450°C in flowing oxygen. Following this treatment the crystals were almost defect free, as determined by Rutherford backscattering-ion channeling measurements. The minimum yields, x,,,. were 2% in the aluminum sublattice and 8% in the oxygen sublattice. Crystals were implanted at 77 K to fluences of 10lh to 10” ions/cm’ using a mass analyzed beam of “Fe (160 kV) with the ion beam = 7” off-normal. The projected range. R p, was 75 nm and range straggling, was 24 nm. Specimens were analyzed by RBS AR,. using 2 MeV He+ ions with a scattering angle of 160°. Mossbauer spectra were obtained using the technique of conversion electron Miissbauer spectroscopy (CEMS). Two important features of CEMS for studies of implanted materials are: (1) CEMS probes only the surface to a depth of 150 to 200 nm. a thickness comparable to the depth of the implanted zone: and (2) aluminum and oxygen produce low photoelectron backgrounds in conversion-electron detection, thus yielding a high signal-to-noise ratio for the implanted iron. In the present study, CEMS spectra were determined at room temperature using a backscattered geometry. The “Co source (100 mCi) was contained in a rhodium matrix and was mounted on a constant acceleration

0 DEPTH

Al DEPTH

lpmi

06

08

1.0

Qun)

t.2

Fig. 1. Ion-channeling results for Al *03 implanted with 4 X 10lh Fe/cm* (160 keV) at 77 K. The analysis was made with 2.0 MeV He+ (160’ scattering angle).

triangular-motion velocity transducer. The data were folded to produce a constant background. The velocity scale and all data are relative to a metallic a-iron absorber at room temperature. The Mijssbauer spectra were fitted to Lorentzian line shapes with a computer least-squares procedure.

3. Results

the

The RBS-ion channeling range of 10lh to 10”

spectra for all fluences in ions/cm’ show that the

aligned yield reaches the random value to a depth of about 170 nm (shown in fig. 1 for 4 x lOI Fe/cm*). The corresponding cross-section TEM micrograph and selected area diffraction pattern (fig. 2) confirmed that the surface was amorphous to a depth of 170 to 180 nm. Similar RBS spectra and TEM results were obtained for fluences of 1 X 10lh. 2 X 10”. 4 x 10”. 7 x 10’h, and 1 x 10” Fe/cm’. The extended electron energy loss fine structure (EXELFS) technique [6] provided a measurement of the aluminum-oxygen first near-neighbor bond lengths for a sample inlplanted with 4 X 10lh Fe/cm’. Fig. 3 shows the first peak in the radial distribution function for the amorphous iron-implanted Al,O,. For comparison, a similar plot for an amorphous Al ?O, layer produced by a stoichiometric implant of 2 Al + 3 0 at 77 K is given. as are the distributions for crystalline a-Al@, and y-Al,O,. The AI-O bond length for the amorphous Fe-implanted material (0.18~5 nm) was significantly different from the amorphous (Al + 0)-implanted material (0.172 nm). The EXELFS results suggest that the SRO in Fe-implanted amorphous samples is similar to that of crystalline ci-Al :O, whereas in stoichiometrically implanted amorphous samples the SRO is similar to that in crystalline y-Al,O,. The CEMS spectra measured at RT for samples implanted with 1, 2, 7. and 10 x 10Jh “Fe/cm* at 77 K are given in fig. 4. The spectrum for the sample implanted with 4 X 10 ” “Fe/cm’ was previously published in ref. [S]. A consistent fit to the spectra was obtained using four doublets and one single line. The Miissbauer parameters are given in table 1.

Fig. 2. Bright-Field image and selected area diffraction pattern for AlzO, implanted (cross-sectioned specimen).

with

4X

10” Fe/cm’

(160 keV) at 77K

Ii. CRYSTALLINE OXIDES / CERAMICS

146

I 10.469nm

I CRYSTALLINE

‘\ . 0.15

0.40

0.20

DISTANCE Fig. 3. Partial

0.25

radial distribution

functions

determined

-

II -6

-4

I -2 VELOCITY

0

2

4

0.25

0.10

6

(mm/s)-+

Fig. 4. Conversion electron MZjssbauer spectra measured at room temperature on Al,O, samples implanted with 160 keV “Fe at 77 K. (a) 1 x 10’” ions/cm’. (b) 2 X 10lh ions/cm2. (c) 7 x 10’” ions/cm’, and (d) 10 X 10” ions/cm’.

0.45 DISTANCE

(nm)

from extended

energy

0.20

0.25

(nm)

loss fine structure (EXELFS) analysis

The single line was ascribed to small metallic iron clusters (Fe”). This line exhibits a negative IS (relative to bulk N-Fe) indicating that the clusters either are y-Fe or are U-Fe under very high compression. The TEM diffraction evidence for 2 nm Fe clusters in c~~v.stu//ine Al,O, implanted with Fe at room temperature suggests the existence of the a-Fe configuration under high isostatic stress [7]. However, the clusters in these amorphous mutrice.r have not yet been detected in the electron microscope and no conclusion can be reached regarding their structure. Two quadrupole doublets were assigned to two different Fe’+ states. In the crystalline Fe-implanted Al,O,, the Fe:+ component was ascribed to a Fe0 (wiistite)-type bond. However, the IS values for the Fe:+ component in the amorphous Fe-implanted Al ,O, are consistently higher and the QS values lower than those for Fe0 [8] or our earlier results [5], leading to an uncertainty in its nature. The parameters for Fef,’ are consistent with a FeA120, spinel-type bond [9] as in the crystalline case. Further TEM and Miissbauer studies will be required in order to complete the description of the local environments and bonding for the Fe” components. The other two doublets have been assigned to two Fe4+ states with high spin configurations. This unusual oxidation state has been observed in nonstoichiometric perovskite-related compounds [10P12] where the Fe4+ occupies a tetragonally distorted octahedral environment. This distortion, an elongation of the FeO, octahedron in one direction, induces the high spin Fe4+ configuration. Such an elongation can be produced by

147 Table 1 Massbauer

parameters

Components

for components

for “Fe

implanted

‘I’

Fluence

Single line (metallic Fe)

IS [mm/s] W [mm/s]

h’

R PI Doublet (Fe;‘+

D,

IS [mm/s]

)

QS [mm/d W [mm/s] R VI

Doublet (Fe?:

Dz

IS [mm/s]

)

QS [mm/d W [mm/s]

2x10’h

7 x 10’h

10 x 10’6

- 0.20 0.49 22

- 0.20 0.49 24

-0.11 0.42 23

-0.10 0.35 21

- 0.09 0.80 0.34 11

-0.10 0.71 0.36 11

0.00 0.72 0.36 29

0.00 0.67 0.34 26

-0.12 0.36 0.24 ”

-0.12 0.38 0.39 14

- 0.07 0.42 0.31 27

0.05 0.38 0.31 33

1.13 2.03 0.56 26

1.29 1.86 0.52 17

1.41 1.53 0.50 9

1.37 1.53 0.52 10

1.16 1 .Ol 0.85 37

1.20 1.26 0.85 33

1.20 1.07 0.67 12

1.17 1.05 0.67 10

4

IS [mm/s]

Doublet D, Fe f,’

QS [mm/s1 W [mm/s] R PI IS [mm/s]

D,

[lons/cm2]

1 x 10’h

R PI

Doublet Fe:+

(160 keV) into Al _,O 3 at 77 K

QS [mm/d W [mm/s] R [%I

‘) IS = isomer shift: QS = quadrupole splitting; I+‘= hne width; and R = relative h’ IS is relative to a-iron at RT. I’ Since the source line width is 0.26 mm/s and the intensity of this component ION 40,

0

FLUENCE 4 I1

2 1

(X40”

8 I

_‘_.*V 20

t

10 I

1

/

01r’

I

I

I

0

2

1

I

40

Fe4+ II

__-*

,/’

ION

ions SCOT?)

6 I

i

I

l

20 t

01

area. is low, large errors FLUENCE 4

in the fit give this low value.

(X (Ot6ions 6

I

40

I

I

.

cm?? 8

I

. .

.

Fe’+ n

x-

I

1

I

I

I

Fe0

(a) n

I

I

I

-0

10

20

30

CONCENTRATION

Fe/Al

PM

0

IO CONCENTRATION

20 Fe/Al

30 f%)

Fig. 5. (a) Fluence dependence of the different components present in the Miissbauer spectra of fig. 4. (b) Fluence dependence components present in the Massbauer spectra of iron-implanted crystalline A120J (300 K) [5].

of the

148

C.J. McHargue

er al. / The sfructure

increasing the covalency of the M-O bonds in the XJ planes [lo]. In some instances, it has been proposed that this highly distorted site arises from the presence of an oxygen vacancy in the octahedron [ll]. The two doublets for Fe4+ in our samples indicate the presence of two slightly different iron environments. In the present case. the Fe4+ ions are in an amorphous phase but we might suppose that the local order consists of distorted Fe4’0, octahedra that contain oxygen vacancies or nonbridging oxygen ions. An intriguing question is whether the Fe4+ distorted octahedra promote the amorphization of AllO, or vice versa. The relative amounts of the five iron states are shown in fig. 5a. There are large differences from our results for the crystalline Fe-implanted AlzO, given in fig. 5b [5]. The relative amount of Fe” was essentially constant (21-240/c) for the fluence range studied for the amorphous samples, whereas there was no iron in this state at fluences below 2 X 10lh Fe/cm’ in the crystalline samples. In the latter samples, the relative amount of Fe” increased to about 50% at 10” Fe/cm’. The variation in the relative amounts of Fe’+ with fluence is similar in the two cases, however, there is much less Fe” at each fluence in the amorphous samples. No Fe4’ was detected in the crystalline samples, but it is the major component at the higher fluences for the amorphous samples. Surprisingly, no Fe3+ was detected in the amorphous samples, whereas crystalline samples may contain as much as 20% of the iron as Fe3+. The previously reported [5] CEMS data for the room temperature implantation of iron into Al z03 were re-examined for evidence of the Fe4+ states. It was found that the multicomponent spectra of the higher fluence samples could be fit by a combination of Fe2’, Fe”. and Fe4+ with no Fe’+. Thus, it is possible that there is a low fluence regime with Fe’+, Fe7’. and Fe’ for the room temperature implantation of iron (i.e., crystalline matrix). The former interpretation is more consistent with the published annealing results [5] but further studies are required in order to definitely confirm the Al,O, and presence of the Fe7+ state in as-implanted the role of the Fe4+ state in the amorphization process.

Implantation of sapphire with 160 keV iron at 77 K produces an amorphous structure for fluences in the

Al,O.,

range of 1 to 10 X 10lh Fe/cm’. The presence of an amorphous state was confirmed by RBS and TEM examination. Short-range order in the amorphous phase was determined by EXELFS analysis and differs from that in amorphous Al,O, produced by stoichiometric aluminum plus oxygen implantation at the same temperature. The CEMS measurements indicate the iron to be present as Fe4+, Fe*+. and Fe”, whereas Fe3+ is absent. The Fe4+ and Fe’+ ions each have two different environments.

References

[l] B.D. Sawicka and J.A. Sawicki, Physics.

vol. 25, ed. U. Comer.

139. [2] A. Perez, G. Mareat, [3]

[4]

[5]

[6]

[7]

[8] [9] [lo] [ll]

4. Summary

ofrmplantrd

[12]

in: Topics in Current (Springer. Berlin, 1981) p.

B.D. Sawicka, J.A. Sawicki and T. Tyliszczak, Phys. Rev. 828 (1983) 1227. J. Kowalski, G. Marest, A. Perez, B.D. Sawicka. J.A. Sawicki, J. Stanek and T. Tyliszczak, Nucl. Inatr. and Meth. 209/210 (1983) 1145. M. Guermazi. G. Marest, A. Perez. B.D. Sawicka, J.A. Sawicki. P. Thevenard and T. Tyliszczak, Mater. Res. Bull. 18 (1983) 529. C.J. McHargue, G.C. Farlow, P.S. Sklad. C.W. White. A. Perez, N. Kornilios and G. Marest, Nucl. Instr. and Meth. B19,‘20 (1987) 813. P.S. Sklad, P. Angelini and J. Sevely. Proc. 46th Ann. Meeting EMSA, ed. G.W. Bailey (San Francisco Press, 1988) p. 468. P.S. Sklad. J.D. McCallum, S.J. Pennycook, C.J. McHargue and C.W. White. in: Characterization of the Structure and Chemistry of Defects in Materials, eds. B.C. Larson. M. Ruhle and D.N. Seidman (Materials Research Society, Pittsburgh, 1989). N.N. Greenwood and T.C. Gihh, in: Mnsshauer Spectroscopy (Chapman and Hall, London, 1971) p. 249. M.J. Rossiter, J. Phys. Chem. Sol. 26 (1965) 775. G. Demazeau. B. Buffat, M. Pouchard and P. Hagenmuller. J. Sol. Stat. Chem. 54 (1984) 389. L. Fournes, Y. Potin, J.C. Grenier. G. Demazeau and M. Pouchard, Sol. Stat. Comm. 62 (1987) 239. G, Demareau, Z. Li-Ming, L. Fournes, M. Pouchard and P. Hagenmuller, J. Sol. Stat. Chem. 72 (1988) 31.